1. Field of the Invention
This invention relates to liquid metal traps or seals. It particularly relates to a filter trap for removing sodium vapor or finely divided sodium particulates from a stream of inert gas. Liquid metal vapor traps are of particular utility for use in liquid metal-cooled nuclear reactors, e.g., sodium-cooled fast breeder reactors. The term sodium vapor as used herein is also intended to include finely divided sodium particulate matter in the form of a fog or aerosol dispersion.
2. Description of the Prior Art
In a nuclear system utilizing liquid sodium as a coolant, circulation of an inert cover gas, such as argon, above the sodium pool leads to evaporation of the sodium. It therefore becomes a necessity to de-entrain the liquid metal coolant from the stream of inert gas to prevent circulation of the liquid metal with the inert gas. Otherwise, when the gas is cooled, deposition of the sodium on system components, such as gas feed pipes, would quickly clog these feed pipes. The purpose of the sodium vapor trap, therefore, is to de-entrain the sodium coolant from the inert gas in such a manner that long term operation of the system can continue without frequent shutdown.
There are a number of prior art vapor traps that serve to remove sodium particulates, as well as particulates of other liquid metals, from an effluent gas. These prior art devices rely primarily on adhesion of the particulates to some type of physical barrier.
A common type of vapor trap utilizes a woven wire filter consisting primarily of a spirally wound, woven wire mesh which is put into a vapor line. The individual layers in the mesh serve to obstruct the passage of sodium entrained in an inert gas. This particular type of vapor trap is very inefficient in that it does not trap all of the sodium in the effluent gas. In addition, the woven wire filter is easily clogged with sodium.
Another type of sodium vapor trap comprises a series of baffles in a container with apertures in each of the baffles, the baffles being stacked parallel to one another with spaces in between. The apertures in the baffles are non-aligned with each other so that the sodium particulate follows a tortuous path through the apertures in the stacked baffles. The sodium entrained in the effluent gases adheres to the surface of the baffle plates, thus removing the sodium from the carrier gases. This type of vapor trap is highly inefficient in that it does not trap all of the sodium in the effluent gases. Additionally, it also becomes easily clogged.
Yet another type of vapor trap utilizes a length of coiled tubing, the sodium vapor being directed to the inside of the coiled tubing in a vapor line. The sodium tends to adhere to the walls of the coiled tube as the vapors follow the tortuous path of the coil. This type of vapor trap is also found lacking in that it, too, can become easily clogged and, like the previously described sodium traps, it does not trap all of the sodium in the effluent gases.
Still another prior art vapor trap consists of a container filled with a multiplicity of "Raschig" rings. Such a trap is generally designated as a condenser vapor trap. The Raschig rings serve to obstruct the sodium in the effluent gases, the sodium adhering to both the outside and inside surfaces of the rings. The containers packed with Raschig rings do not remove all of the sodium in the effluent gases emanating from the molten sodium bath adjacent to the trap. Also, if improperly operated relative to its temperature distribution, the container can easily become clogged. The above four devices may be termed "depth processing" traps since they depend for operation on condensation of the particulates as the particulates incrementally pass through layer after layer of filter material.
An encouraging method for sodium particulate removal is one which involves a "surface process" of sodium particulate removal from the effluent gases. Such vapor traps utilize a series of stacked, sintered, porous discs, cylinders or other geometries in a sodium vapor line. These hollow stacked discs serve to de-entrain the vaporized or finely dispersed sodium from the effluent gases, primarily on the disc surfaces as the gases pass through the sintered disc stack. This type of filter vapor trap removes virtually all of the sodium particulates from the effluent gases by the build up of a cake layer of frozen sodium particles on the surface of the discs. However, as the cake builds in thickness, the cake collapses, compacting itself against the sintered discs, and with time blocking or clogging virtually all of the interstices in the disc, thereby preventing gas flow therethrough.